The research in this proposal is aimed at further elucidating the mechanisms of synaptic vesicle exocytosis, and the mechanisms by which it is regulated. The studies will focus on release of glutamate from hippocampal neurons in culture, but it is anticipated that they will have broader relevance, because of the basic level at which the problem is being approached. Synaptic vesicles have been hypothesized to undergo exocytosis via two mechanisms;full fusion and collapse into the plasma membrane, and release of transmitter through a small fusion pore. The very different characteristics of these two modes are likely to profoundly influence the properties of synaptic transmission at a given synapse. Recent evidence indicates that both mechanisms may contribute to release at central synapses, leading to the further questions of the implications of these modes of release for physiological function, and how this might be regulated. The proposed research has three broad objectives;to determine the balance between fusion pore mediated, and full-fusion mediated exocytosis at CA1 hippocampal synapses, secondly, to determine how this balance is regulated, and, thirdly, to understand the physiological consequences. Preliminary data has indicated that both exocytic modes are represented at this synapse, and that exocytosis via a fusion pore mechanism leads to greatly reduced postsynaptic activation. Further investigation using both synaptic and modeling methods will establish whether this in fact results in postsynaptic receptor desensitization. The principle candidate for regulation of the balance between the two modes of exocytosis is intra-terminal calcium ion concentration. For this reason a major aim of this study is seeking to understand the way in which calcium acts in presynaptic terminals. The nervous system represents the highest level of regulation for the functioning of the body. In addition to higher functions, this includes control of movement and also regulation of physiological systems through the sympathetic and parasympathetic nervous systems;consequently a full understanding of the mechanisms which underlie synaptic transmission is highly relevant to research efforts aimed at understanding and curing human disease. This grant seeks to use recent advances in the study of synaptic transmission to extend our understanding of the operation of the nervous system in health and disease.